Hematopoietic defects in response to reduced Arhgap21

RHO subfamily of small GTPases in the development and function of hematopoietic cells

RHOA, RHOB, and RHOC comprise a subfamily of RHO GTPase proteins famed for controlling cytoskeletal dynamics. RHO proteins operate downstream of multiple signals emerging from the microenvironment, leading to diverse cell responses, such as proliferation, adhesion, and migration. Therefore, RHO signaling has been centrally placed in the regulation of blood cells. Despite their high homology, unique roles of RHOA, RHOB, and RHOC have been described in hematopoietic cells. In this article, we overview the contribution of RHO proteins in the development and function of each blood cell lineage. Additionally, we highlight the aberrations of the RHO signaling pathways found in hematological malignancies, providing clues for the identification of new therapeutic targets.

Thrombopoietin treats erythropoietin resistance by correcting EPO-induced progenitorcell depletion

Recombinant human erythropoietin (rHuEPO) is a prevalent treatment for anemia in patients with chronic kidney disease. However, up to 10% of these patients exhibit EPO resistance or hyporesponsiveness, which may be caused by the depletion of erythroid progenitor cells. Thrombopoietin (TPO) has the potential to promote the growth of early progenitor cells and correct the depletion. In this study, we investigate the efficacy and the underlying mechanism of the combination therapy of TPO and EPO to EPO resistance. First, the in vivo studies suggested that intensive EPO treatment induced progenitor cell depletion in the bone marrow, where the depletion was corrected by TPO. Then, colony assays showed that EPO and TPO synergistically enhanced the burst-forming unit-erythroid (BFU-E) production but antagonistically boosted the colony-forming units of megakaryocytes (CFU-MK) production. Also, we found TPO promoted hematopoietic stem and progenitor cells (HSPCs) production, while EPO drove HSPCs toward the erythroid lineage. Additionally, EPO induced more megakaryocytic-erythroid progenitors (MEPs) toward the erythroid output. Model-based simulations indicate the efficacy of this combination therapy for treating EPO-resistant anemia in rats. In conclusion, our study demonstrated the efficacy of combination therapy in addressing EPO-resistant anemia by correcting EPO-induced erythroid progenitor depletion.

A Structural Network Analysis of Neuronal ArhGAP21/23 Interactors by Computational Modeling

RhoGTPase-activating proteins (RhoGAPs) play multiple roles in neuronal development; however, details of their substrate recognition system remain elusive. ArhGAP21 and ArhGAP23 are RhoGAPs that contain N-terminal PDZ and pleckstrin homology domains. In the present study, the RhoGAP domain of these ArhGAPs was computationally modeled by template-based methods and the AlphaFold2 software program, and their intrinsic RhoGTPase recognition mechanism was analyzed from the domain structures using the protein docking programs HADDOCK and HDOCK. ArhGAP21 was predicted to preferentially catalyze Cdc42, RhoA, RhoB, RhoC, and RhoG and to downregulate RhoD and Tc10 activities. Regarding ArhGAP23, RhoA and Cdc42 were deduced to be its substrates, whereas RhoD downregulation was predicted to be less efficient. The PDZ domains of ArhGAP21/23 possess the FTLRXXXVY sequence, and similar globular folding consists of antiparalleled β-sheets and two α-helices that are conserved with PDZ domains of MAST-family proteins. A peptide docking analysis revealed the specific interaction of the ArhGAP23 PDZ domain with the PTEN C-terminus. The pleckstrin homology domain structure of ArhGAP23 was also predicted, and the functional selectivity for the interactors regulated by the folding and disordered domains in ArhGAP21 and ArhGAP23 was examined by an in silico analysis. An interaction analysis of these RhoGAPs revealed the existence of mammalian ArhGAP21/23-specific type I and type III Arf- and RhoGTPase-regulated signaling. Multiple recognition systems of RhoGTPase substrates and selective Arf-dependent localization of ArhGAP21/23 may form the basis of the functional core signaling necessary for synaptic homeostasis and axon/dendritic transport regulated by RhoGAP localization and activities.

Association analysis of polymorphisms of candidate genes for laying traits in Yangzhou geese

Egg production is an important economic trait in the Chinese goose industry. Due to the low heritability of annual egg production traits in geese, large-scale individual selection based on annual egg production measurements cannot be carried out. Therefore, new selection methods must be applied for large-scale early selections. To screen for effective molecular markers for early Yangzhou geese selection, the genotypes and gene frequencies of mutated loci of five candidate genes related to egg production, MAGI-1, ACSF2, ASTN2, KIAA1462, and ARHGAP21, were detected and analyzed by PCR-direct sequencing.Furthermore, correlation analysis was performed with annual egg mass and body weight at the point of lay and egg weight, and the results were as follows:Magi-1 (Record-106975)was A > G, ACSF2 (Record-106582)was A > C, ASTN2 (Record-111407)was A > T, KIAA1462 (Record-134172)was A > T, and the base of ARHGAP21 (Record-112359) was G > T. At all the five loci above, the Yangzhou geese population followed the Hardy-Weinberg equilibrium (P > 0.05). The results of the association analysis between different genotypes and production performance showed no significant differences in annual egg production, body weight at the point of lay, and egg weight, among different genotypes (P > 0.05) at the mutation loci of MAGI-1 and ASTN2. At the ACSF2 and KIAA1462, the annual egg production of AC was significantly higher than that of AA and CC (P < 0.05), the annual egg production of TT was significantly higher than that of AA (P < 0.05), and there were no significant differences in body weight at the point of lay and egg weight, among the three genotypes (P > 0.05). At ARHGAP21, the body weight at the lay point of the TT genotype was the highest, which was significantly higher than that of GG (P < 0.05); however, there was no significant difference with the heterozygous GT genotype for this trait (P > 0.05). Therefore, Genotype AC at ACSF2 and genotype TT at KIAA1462 could be used as favorable genotypes for egg production, and genotype TT at ARHGAP21 could be used as a favorable genotype for weight in Yangzhou geese.

Turning Platelets Off and On: Role of RhoGAPs and RhoGEFs in Platelet Activity

Platelet cytoskeletal reorganisation is a critical component of platelet activation and thrombus formation in haemostasis. The Rho GTPases RhoA, Rac1 and Cdc42 are the primary drivers in the dynamic reorganisation process, leading to the development of filopodia and lamellipodia which dramatically increase platelet surface area upon activation. Rho GTPases cycle between their active (GTP-bound) and inactive (GDP-bound) states through tightly regulated processes, central to which are the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs catalyse the dissociation of GDP by inducing changes in the nucleotide binding site, facilitating GTP binding and activating Rho GTPases. By contrast, while all GTPases possess intrinsic hydrolysing activity, this reaction is extremely slow. Therefore, GAPs catalyse the hydrolysis of GTP to GDP, reverting Rho GTPases to their inactive state. Our current knowledge of these proteins is constantly being updated but there is considerably less known about the functionality of Rho GTPase specific GAPs and GEFs in platelets. In the present review, we discuss GAP and GEF proteins for Rho GTPases identified in platelets, their regulation, biological function and present a case for their further study in platelets.

Arhgap21 Deficiency Results in Increase of Osteoblastic Lineage Cells in the Murine Bone Marrow Microenvironment

ARHGAP21 is a member of the RhoGAP family of proteins involved in cell growth, differentiation, and adhesion. We have previously shown that the heterozygous Arhgap21 knockout mouse model (Arhgap21^+/-) presents several alterations in the hematopoietic compartment, including increased frequency of hematopoietic stem and progenitor cells (HSPC) with impaired adhesion in vitro, increased mobilization to peripheral blood, and decreased engraftment after bone marrow transplantation. Although these HSPC functions strongly depend on their interactions with the components of the bone marrow (BM) niche, the role of ARHGAP21 in the marrow microenvironment has not yet been explored. In this study, we investigated the composition and function of the BM microenvironment in Arhgap21^+/- mice. The BM of Arhgap21^+/- mice presented a significant increase in the frequency of phenotypic osteoblastic lineage cells, with no differences in the frequencies of multipotent stromal cells or endothelial cells when compared to the BM of wild type mice. Arhgap21^+/- BM cells had increased capacity of generating osteogenic colony-forming units (CFU-OB) in vitro and higher levels of osteocalcin were detected in the Arhgap21^+/- BM supernatant. Increased expression of Col1a1, Ocn and decreased expression of Trap1 were observed after osteogenic differentiation of Arhgap21^+/- BM cells. In addition, Arhgap21^+/- mice recipients of normal BM cells showed decreased leucocyte numbers during transplantation recovery. Our data suggest participation of ARHGAP21 in the balanced composition of the BM microenvironment through the regulation of osteogenic differentiation.

Role of Rho GTPases in stem cell regulation

The future of regenerative medicine relies on our understanding of stem cells which are essential for tissue/organ generation and regeneration to maintain and/or restore tissue homeostasis. Rho family GTPases are known regulators of a wide variety of cellular processes related to cytoskeletal dynamics, polarity and gene transcription. In the last decade, major new advances have been made in understanding the regulatory role and mechanism of Rho GTPases in self-renewal, differentiation, migration, and lineage specification in tissue-specific signaling mechanisms in various stem cell types to regulate embryonic development, adult tissue homeostasis, and tissue regeneration upon stress or damage. Importantly, implication of Rho GTPases and their upstream regulators or downstream effectors in the transformation, migration, invasion and tumorigenesis of diverse cancer stem cells highlights the potential of Rho GTPase targeting in cancer therapy. In this review, we discuss recent evidence of Rho GTPase signaling in the regulation of embryonic stem cells, multiple somatic stem cells, and cancer stem cells. We propose promising areas where Rho GTPase pathways may serve as useful targets for stem cell manipulation and related future therapies.

The role of ARHGAP9: clinical implication and potential function in acute myeloid leukemia

Background

Rho GTPase activating protein 9 (ARHGAP9) is expressed in various types of cancers and can inactivate Rho GTPases that mainly regulate cytoskeletal dynamics. However, the exact role of ARHGAP9 in acute myeloid leukemia (AML) has yet to be clarified.

Methods

We compared the transcriptional expression, prognosis, differentially expressed genes, functional enrichment, and hub genes in AML patients on the basis of the data published in the following databases: UALCAN, GEPIA, Gene Expression Omnibus, the Human Protein Atlas, Cancer Cell Line Encyclopedia, LinkedOmics, Metascape, and String. Data from the Cancer Genome Atlas database was used to evaluate the correlations between ARHGAP9 expression and various clinicopathological parameters, as well as the significantly different genes associated with ARHGAP9 expression.

Results

We found that ARHGAP9 expression was higher in the tissues and cell lines extracted from patients with AML than corresponding control tissues and other cancer types. ARHGAP9 overexpression was associated with decreased overall survival (OS) in AML. Compared with the ARHGAP9^low group, the ARHGAP9^high group, which received only chemotherapy, showed significantly worse OS and event-free survival (EFS); however, no significant difference was observed after treatment with autologous or allogeneic hematopoietic stem cell transplantation (auto/allo-HSCT). The ARHGAP9^high patients undergoing auto/allo-HSCT also had a significantly better prognosis with respect to OS and EFS than those receiving only chemotherapy. Most overlapping genes of the significantly different genes and co-expression genes exhibited enriched immune functions, suggesting the immune regulation potential of ARHGAP9 in AML. A total of 32 hub genes were identified from the differentially expressed genes, within which the KIF20A had a significant prognostic value for AML.

Conclusions

ARHGAP9 overexpression was associated with poor OS in AML patients and can be used as a prognostic biomarker. AML patients with ARHGAP9 overexpression can benefit from auto/allo-HSCT rather than chemotherapy.

Hes1 deficiency causes hematopoietic stem cell exhaustion

The transcriptional repressor Hairy Enhancer of Split 1 (HES1) plays an essential role in the development of many organs by promoting the maintenance of stem/progenitor cells, controlling the reversibility of cellular quiescence, and regulating both cell fate decisions. Deletion of Hes1 in mice results in severe defects in multiple organs and is lethal in late embryogenesis. Here we have investigated the role of HES1 in hematopoiesis using a hematopoietic lineage‐specific Hes1 knockout mouse model. We found that while Hes1 is dispensable for steady‐state hematopoiesis, Hes1‐deficient hematopoietic stem cells (HSCs) undergo exhaustion under replicative stress. Loss of Hes1 upregulates the expression of genes involved in PPARγ signaling and fatty acid metabolism pathways, and augments fatty acid oxidation (FAO) in Hes1^f/fVav1Cre HSCs and progenitors. Functionally, PPARγ targeting or FAO inhibition ameliorates the repopulating defects of Hes1^f/fVav1Cre HSCs through improving quiescence in HSCs. Lastly, transcriptome analysis reveals that disruption of Hes1 in hematopoietic lineage alters expression of genes critical to HSC function, PPARγ signaling, and fatty acid metabolism. Together, our findings identify a novel role of HES1 in regulating stress hematopoiesis and provide mechanistic insight into the function of HES1 in HSC maintenance.

Low iron promotes megakaryocytic commitment of megakaryocytic-erythroid progenitors in humans and mice

The mechanisms underlying thrombocytosis in patients with iron deficiency anemia remain unknown. Here, we present findings that support the hypothesis that low iron biases the commitment of megakaryocytic (Mk)-erythroid progenitors (MEPs) toward the Mk lineage in both human and mouse. In MEPs of transmembrane serine protease 6 knockout (Tmprss6-/-) mice, which exhibit iron deficiency anemia and thrombocytosis, we observed a Mk bias, decreased labile iron, and decreased proliferation relative to wild-type (WT) MEPs. Bone marrow transplantation assays suggest that systemic iron deficiency, rather than a local role for Tmprss6-/- in hematopoietic cells, contributes to the MEP lineage commitment bias observed in Tmprss6-/- mice. Nontransgenic mice with acquired iron deficiency anemia also show thrombocytosis and Mk-biased MEPs. Gene expression analysis reveals that messenger RNAs encoding genes involved in metabolic, vascular endothelial growth factor, and extracellular signal-regulated kinase (ERK) pathways are enriched in Tmprss6-/- vs WT MEPs. Corroborating our findings from the murine models of iron deficiency anemia, primary human MEPs exhibit decreased proliferation and Mk-biased commitment after knockdown of transferrin receptor 2, a putative iron sensor. Signal transduction analyses reveal that both human and murine MEP have lower levels of phospho-ERK1/2 in iron-deficient conditions compared with controls. These data are consistent with a model in which low iron in the marrow environment affects MEP metabolism, attenuates ERK signaling, slows proliferation, and biases MEPs toward Mk lineage commitment.

The Molecular Signature of Megakaryocyte-Erythroid Progenitors Reveals a Role for the Cell Cycle in Fate Specification

Megakaryocytic-erythroid progenitors (MEPs) give rise to the cells that produce red blood cells and platelets. Although the mechanisms underlying megakaryocytic (MK) and erythroid (E) maturation have been described, those controlling their specification from MEPs are unknown. Single-cell RNA sequencing of primary human MEPs, common myeloid progenitors (CMPs), megakaryocyte progenitors, and E progenitors revealed a distinct transitional MEP signature. Inferred regulatory transcription factors (TFs) were associated with differential expression of cell cycle regulators. Genetic manipulation of selected TFs validated their role in lineage specification and demonstrated coincident modulation of the cell cycle. Genetic and pharmacologic modulation demonstrated that cell cycle activation is sufficient to promote E versus MK specification. These findings, obtained from healthy human cells, lay a foundation to study the mechanisms underlying benign and malignant disease states of the megakaryocytic and E lineages.

Bipotent megakaryocytic-erythroid progenitors (MEPs) produce megakaryocytic and erythroid cells. Using single-cell RNA sequencing of primary human MEPs and their upstream and downstream progenitors, Lu et al. show that MEPs are a unique transitional population. Functional and molecular studies show that MEP lineage fate is toggled by cell cycle speed.

ARHGAP21 deficiency impairs hepatic lipid metabolism and improves insulin signaling in lean and obese mice

ARHGAP21 is a Rho-GAP that controls GTPases activity in several tissues, but its role on liver lipid metabolism is unknown. Thus, to achieve the Rho-GAP role in the liver, control and ARHGAP21-haplodeficient mice were fed chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD) for 12 weeks, and pyruvate and insulin tolerance tests, insulin signaling, liver glycogen and triglycerides content, gene and protein expression, and very-low-density lipoprotein secretion were measured. Het mice displayed reduced body weight and plasma triglycerides levels, and increased liver insulin signaling. Reduced gluconeogenesis and increased glycogen content were observed in Het-HFD mice. Gene and protein expression of microsomal triglyceride transfer protein were reduced in both Het mice, while the lipogenic genes SREBP-1c and ACC were increased. ARHGAP21 knockdown resulted in hepatic steatosis through increased hepatic lipogenesis activity coupled with decreases in CPT1a expression and very-low-density lipoprotein export. In conclusion, liver of ARHGAP21-haplodeficient mice are more insulin sensitive, associated with higher lipid synthesis and lower lipid export.

Transmembrane Protein Aptamer Induces Cooperative Signaling by the EPO Receptor and the Cytokine Receptor β-Common Subunit

The erythropoietin receptor (EPOR) plays an essential role in erythropoiesis and other cellular processes by forming distinct signaling complexes composed of EPOR homodimers or hetero-oligomers between the EPOR and another receptor, but the mechanism of heteroreceptor assembly and signaling is poorly understood. We report here a 46-residue, artificial transmembrane protein aptamer, designated ELI-3, that binds and activates the EPOR and induces growth factor independence in murine BaF3 cells expressing the EPOR. ELI-3 requires the transmembrane domain and JAK2-binding sites of the EPOR for activity, but not the cytoplasmic tyrosines that mediate canonical EPOR signaling. Instead, ELI-3-induced proliferation and activation of JAK/STAT signaling requires the transmembrane and cytoplasmic domains of the cytokine receptor β-common subunit (βcR) in addition to the EPOR. Moreover, ELI-3 fails to induce erythroid differentiation of primary human hematopoietic progenitor cells but inhibits nonhematopoietic cell death induced by serum withdrawal.

Systematic review of hormonal and genetic factors involved in the nonsyndromic disorders of the lower jaw

Mandibular disorders are among the most common birth defects in humans, yet the etiological factors are largely unknown. Most of the neonates affected by mandibular abnormalities have a sequence of secondary anomalies, including airway obstruction and feeding problems, that reduce the quality of life. In the event of lacking corrective surgeries, patients with mandibular congenital disorders suffer from additional lifelong problems such as sleep apnea and temporomandibular disorders, among others. The goal of this systematic review is to gather evidence on hormonal and genetic factors that are involved in signaling pathways and interactions that are potentially associated with the nonsyndromic mandibular disorders. We found that members of FGF and BMP pathways, including FGF8/10, FGFR2/3, BMP2/4/7, BMPR1A, ACVR1, and ACVR2A/B, have a prominent number of gene-gene interactions among all identified genes in this review. Gene ontology of the 154 genes showed that the functional gene sets are involved in all aspects of cellular processes and organogenesis. Some of the genes identified by the genome-wide association studies of common mandibular disorders are involved in skeletal formation and growth retardation based on animal models, suggesting a potential direct role as genetic risk factors in the common complex jaw disorders. Developmental Dynamics 248:162-172, 2019. © 2018 Wiley Periodicals, Inc.

Whole-Body ARHGAP21-Deficiency Improves Energetic Homeostasis in Lean and Obese Mice

Inhibition of Rab-GAP TBC1 domain family member 1 (TBC1D1) reduces body weight and increases energy expenditure in mice. Here, we assessed the possible involvement of GTPase activating protein 21 (ARHGAP21), a Rho-GAP protein, in energy homeostasis. Wild-type and whole-body ARHGAP21-haplodeficient mice were fed either chow or high-fat diet for 10 weeks. These mice were analyzed for body weight, food intake, voluntary physical activity, and energy expenditure by indirect calorimetry. Real-time PCR was performed to determine changes in the expression of hypothalamic-anorexic genes. Whole-body ARHGAP21-haplodeficient mice showed lower body weight and food intake associated with increased energy expenditure. These mice also showed higher expression of hypothalamic-anorexic genes such as POMC and CART. Our data suggest that the reduction in body weight of ARHGAP21-haplodeficient mice was related to alterations in the central nervous system. This suggests a new role for ARHGAP21 in energetic metabolism and prompts us to consider GAP protein members as possible targets for the prevention and treatment of obesity and related diseases.

The neglected terminators: Rho family GAPs in neutrophils

BACKGROUND

GTPase-activating proteins (GAPs) accelerate the rate of hydrolysis of GTP bound to small GTPases, thereby limiting the prevalence and concentration of the active, GTP-bound form of these proteins. The large number of potential GAPs acting on members of the Rho family of small GTPases raises the question of specificity or redundancy.

RESULTS

In this review, we summarize experimental data obtained on the role of Rho family GAPs in neutrophils, highlight cases where more than one GAP is involved in a physiological function and show examples that GAPs can be involved not only in termination but also in initiation of cellular processes. We demonstrate that the expression-level regulation of GAPs may also occur in short-living cells such as neutrophils. Finally, we provide insight into the existence and structure of molecular complexes in which Rho family GAPs are involved.

CONCLUSION

GAPs play more complex and varied roles than being simple terminators of cellular processes.

Concise Review: Bipotent Megakaryocytic-Erythroid Progenitors: Concepts and Controversies

Hematopoietic stem and progenitor cells maintain blood formation throughout our lifetime by undergoing long- and short-term self-renewal, respectively. As progenitor cells progress through the hematopoiesis process, their differentiation capabilities narrow, such that the precursors become committed to only one or two lineages. This Review focuses on recent advances in the identification and characterization of bipotent megakaryocytic-erythroid progenitors (MEP), the cells that can further produce two completely different functional outputs: platelets and red blood cells. The existence of MEP has sparked controversy as studies describing the requirement for this intermediate progenitor stage prior to commitment to the erythroid and megakaryocytic lineages have been potentially contradictory. Interpretation of these studies is complicated by the variety of species, cell sources, and analytical approaches used along with inherent challenges in the continuum of hematopoiesis, where hematopoietic progenitors do not stop at discrete steps on single paths as classically drawn in hematopoietic hierarchy models. With the goal of improving our understanding of human hematopoiesis, we discuss findings in both human and murine cells. Based on these data, MEP clearly represent a transitional stage of differentiation in at least one route to the generation of both megakaryocytes and erythroid cells. Stem Cells 2018;36:1138-1145.

Whole body ARHGAP21 reduction improves glucose homeostasis in high-fat diet obese mice

GTPase activating proteins (GAPs) are ubiquitously expressed, and their role in cellular adhesion and membrane traffic processes have been well described. TBC1D1, which is a Rab-GAP, is necessary for adequate glucose uptake by muscle cells, whereas increased TCGAP, which is a Rho-GAP, decreases GLUT4 translocation, and consequently glucose uptake in adipocytes. Here, we assessed the possible involvement of ARHGAP21, a Rho-GAP protein, in glucose homeostasis. For this purpose, wild type mice and ARHGAP21 transgenic whole-body gene-deficiency mice (heterozygous mice, expressing approximately 50% of ARHGAP21) were fed either chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD). Het-HFD mice showed a reduction in white fat storage, reflected in a lower body weight gain. These mice also displayed an improvement in insulin sensitivity and glucose tolerance, which likely contributed to reduced insulin secretion and pancreatic beta cell area. The reduction of body weight was also observed in Het mice and this phenomenon was associated with an increase in brown adipose tissue and reduced muscle weight, without alteration in glucose-insulin homeostasis. In conclusion, the whole body ARHGAP21 reduction improved glucose homeostasis and protected against diet-induced obesity specifically in Het-HFD mice. However, the mechanism by which ARHGAP21 leads to these outcomes requires further investigation.